1. Field of the Invention
The present invention relates to a voltage conversion device, and more particularly, to a voltage conversion device having non-linear gain and changeable gain polarity.
2. Description of the Prior Art
A liquid crystal display (LCD) monitor has characteristics of light shape, low power consumption, zero radiation, etc. and has been widely used in many information technology (IT) products, such as computer systems, mobile phones, and personal digital assistants (PDAs). The operating principle of the LCD is based on a property that liquid crystals in different twist status can exert different polarization and refraction effects on light. Thus, the liquid crystals arranged in different twist status control penetration amount of light so that various intensity of output light and red, green and blue lights in diverse gray levels can be produced.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a thin-film-transistor (TFT) LCD monitor 10 according to the prior art. The LCD monitor 10 includes an LCD panel 100, a control circuit 102, a data-line-signal output circuit 104, a scan-line-signal output circuit 106, and a voltage generator 108. The LCD panel 100 is formed with two substrates, and there are LCD layers stuffed between the substrates. One substrate includes a plurality of data lines 110, a plurality of scan lines (or gate lines) 112 vertical to the data lines 110, and a plurality of TFTs 114. The other substrate includes a common electrode for providing a common voltage Vcom generated by the voltage generator 108. For the sake of brevity, FIG. 1 only reveals four TFTs 114, but in a real case, each of TFTs 114 is set at an intersection of a data line 110 and a scan line 112 on the LCD panel 100. In other words, the plurality of TFTs 114, each corresponding to a pixel, form a matrix on the LCD panel 100, and thereby the data lines 110 and the scan lines 112 are corresponding to columns and rows of the matrix. In addition, a circuit effect resulted from the two substrates of the LCD panel 100 can be regarded as equivalent capacitors 116.
A driving process of the prior art TFT LCD monitor 10 is described in detail as follows. When the control circuit 102 receives a horizontal synchronization signal 118 and a vertical synchronization signal 120, the control circuit 102 generates corresponding control signals for the data-line-signal output circuit 104 and the scan-line-signal output circuit 106. The data-line-signal output circuit 104 and the scan-line-signal output circuit 106 generate input signals for the data lines 110 and the scan lines 112 according to the control signals, in order to control the TFTs 114 and voltage differences of the equivalent capacitors 116. The voltage differences change twist of liquid crystals and corresponding penetration amount of light, so as to display the display data 122 on a panel. For example, the scan-line-signal output circuit 106 outputs a pulse wave for turning on a TFT 114, and signals of a corresponding data line 110 outputted from the data-line-signal output circuit 104 can pass through the TFT 114 to a corresponding equivalent capacitor 116, so as to control a gray level of corresponding pixel. Besides, controlling signal levels of the signals on the data line 110 outputted from the data-line-signal output circuit 104 can drive gray levels of corresponding pixels.
In the prior art, in order to reduce EMI and save power under high-frequency operations, voltage swing outputted from the display data 122 is expected to be small, i.e., 0.1V˜1V. Therefore, signals inputted to the data-line-signal output circuit 104 need to pass through a voltage conversion circuit, which adjusts the voltage swing of the inputted signals to a predetermined range, so as to control chrominance and luminance of corresponding pixels. For example, please refer to FIG. 2, which is a schematic diagram of a voltage conversion circuit 20 according to the prior art. The voltage conversion circuit 20 includes an operational amplifier 200 and resistors 202, 204. The operational amplifier 200 has an input end 206 coupled to an output end of the display data 122, and an output end 208 coupled to an input end of the data-line-signal output circuit 104. As those skilled in the art recognized, if the operational amplifier 200 is an ideal operational amplifier, which has an infinite gain, and resistance of the resistors 202, 204 are R1 and R2, the gain of the voltage conversion circuit 20 can be derived as a value, (1+R2/R1). In other words, the relation between the input and output voltages of the voltage conversion circuit 20 is corresponding to a linear function.
Therefore, data signals outputted from the display data 122 are amplified via the voltage conversion circuit 20 and then are outputted to the data-line-signal output circuit 104. Under this circumstance, via the voltage conversion circuit 20 of the prior art, data-line signals outputted from the data-line-signal output circuit 104 are linearly amplified to an operating range of the TFTs 114, so that the relation between the gray level and brightness displayed on the TFT LCD monitor 10 is approximate to a linear line, which effects color gradient of images and results in unnatural display images.
Besides, regarding LCD monitors, an event related to necessity of inverting polarity of output voltage also has to be taken into consideration. For an LCD monitor, continuously using a positive or negative polarity voltage to drive liquid crystals will decrease ability of the liquid crystals to polarize or reflect light, which makes image quality worse. Thus, in order to prevent the liquid crystals from breakage resulted from the driving voltage, using the positive and negative polarity voltages to alternately drive the liquid crystals is required. Furthermore, in addition to the equivalent capacitors, the circuit itself produces parasitic capacitors. When an image is displayed on the LCD panel for a long time, the parasitic capacitors may result in a residual image effect due to storage charge and further affect the follow-up image displaying. Therefore, the positive and negative polarity voltages must drive the liquid crystals in an alternate manner, such as methods of line inversion, dot inversion, and etc., so as to improve the effect mentioned above. That is to say, polarity of the data-line signals outputted from the data-line-signal output circuit can be inverted timely to prevent from the residual image effect or breakage of the liquid crystals.
Therefore, the voltage conversion circuit 20 of the prior art linearly amplifies the signals outputted by the display data 122, so that the relation between the gray levels and brightness displayed by the TFT LCD monitor 10 is approximate to a linear function, resulting in bad color gradient and unnatural images. Besides, the voltage conversion circuit 20 cannot provide a driving operation of alternating the positive and negative polarity voltages for inverting the data-line signals outputted from the data-line-signal output circuit 104. Therefore, problems of the residual image effect and breakage of the liquid crystals may easily occur in the TFT LCD monitor 10.